Ribner oversees the isolation unit. He says one of the patients was expected to arrive Monday, while a second was expected several days later.

The Pentagon's press secretary, Navy Rear Adm. John Kirby, said Friday that private-chartered aircraft will be arriving at Dobbins Air Reserve Base in metro Atlanta with patients evacuated from Africa. Kirby did not have further details.

One plane left Cartersville, Georgia Thursday and was headed to Liberia, where the American victims are fighting the viral infection.

According to the CDC, the plane would be equipped with an Aeromedical Biological Containment System (ABCS). The agency says the system is a "portable, tent-like device" that can be installed in a modified Gulfstream plane.

Aeromedical Biological Containment System

CDC/Division of Emergency Operations

"It provides a means to perform emergency movement of exposed or contagious CDC personnel from the field, or site of exposure, to a facility that can provide appropriate medical care without risk to passengers or air crew," according to a statement from the agency's Division of Emergency Operations.

Samaritan's Purse, the relief agency where Brantly works, released a statement Friday saying the two Americans remain in Liberia but "medical evacuation efforts are underway and should be completed by early next week."

CBS News correspondent Vicente Arenas reports that Emory University Hospital has a special ward meant for CDC employees who contract dangerous communicable diseases while working overseas -- just like the two American missionaries now in a life-and-death struggle with the Ebola virus.

Emory's isolation unit is on the ground floor but physically separated from other wards, Arenas reported.

Brantly and Writebol are in stable but serious condition. Brantly "took a slight turn for the worse" Wednesday night, according to Samaritan's Purse.

In an effort to boost their ability to fight the virus, both were given antibodies from Ebola survivors. Writebol received them from an experimental serum, Brantly, from one of his patients, a 14-year-old-boy who lived.

"It sounds like their condition is deteriorating and there is medical care that can be given here in the sophisticated health settings that we have here that simply can't be done in Africa," said CBS News chief medical correspondent Dr. Jon LaPook.

For example, LaPook said, the victims who are experiencing blood clots can be given fresh frozen plasma.

LaPook said that experimental vaccines are in the works - NIH has a vaccine that is in the first phase of testing - but nothing is "available for prime time any time real soon."

Brantly's family has shown no symptoms of the deadly disease. His wife Amber and 3- and 5-year-old children left Liberia days before Brantly quarantined himself after testing positive for Ebola, raising fears they may have been infected.

"We don't really know how effective the serum will be in this instance of Ebola because it really hasn't been done very frequently before, if at all," said Dr. William Schaffner, of Vanderbilt University Medical Center.

On Thursday, the CDC issued a level-3 travel warning -- the highest possible. The agency said Americans should avoid all non-essential travel to Sierra Lone, Guinea and Liberia, where more than 700 people have already died.

"The bottom line is Ebola is worsening in West Africa," said Dr. Tom Frieden, the CDC director who announced the travel warning.

He called Ebola "a tragic, dreadful and merciless virus."

Passengers arriving in the U.S. from West Africa are currently not being screened. But public health officials have put hospitals on alert. They say any outbreaks will be quickly brought under control.

Emory's three-bed isolation center has highly trained staff and specialized equipment designed to keep dangerous pathogens from getting out, Arenas reported. It is one of only four such facilities in the entire country.

The CDC has about two dozen staffers in West Africa to help try to control the outbreak. Frieden on Thursday said the CDC will send 50 more in the next month. CDC workers in Africa also are helping at airports to help screen passengers, he said.

The CDC has said that the risk of a traveler bringing the Ebola virus to the United States remains small. On Monday, the agency sent a health alert to U.S. doctors, updating them about the outbreak. The alert stressed they should ask about foreign travel in patients who come down with Ebola-like symptoms, including fever, headache, vomiting and diarrhea.

Even if a traveler infected with Ebola came to the U.S., the risk of an outbreak is considered very low, Frieden said. Patients are contagious only when they show symptoms and U.S. hospitals are well equipped to isolate cases and control spread of the virus.

Frieden also noted that relatively few people travel from West Africa to the United States. He said about 10,000 travelers from those countries come to the United States in an average three- or four-month period, and most do not arrive on direct flights.

The CDC has staff at 20 U.S. airports and border crossings. They evaluate any travelers with signs of dangerous infectious diseases, and isolate them when necessary. The agency is prepared to increase that staffing if needed, he said.

Frieden said a widespread Ebola outbreak in the United States "is not in the cards."